Single state machine across main and object stores

[badrishc]

Changes:

• Share Epoch and StateMachineDriver (SMD) across main and object stores
• Garnet checkpointing uses this shared driver to checkpoint both stores with a single state machine
• Transactions use SMD to register transactions, so that a single transaction version is maintained across the two stores.
• Transactions no longer barrier during the (v) -> (v+1) switchover. We ensure correctness by verifying the transaction version after all locks are acquired.
• Move NumActiveLockingSessions (renamed to NumActiveTransactions) to SMD and ensure correct (v) -> (v+1) switchover with active transactions.
• Removed targetVersion concept so that checkpoint versions always increment in steps of 1.
• Refactored CheckpointVersionShift to a pair of CheckpointVersionShiftStart and CheckpointVersionShiftEnd to help demarcate start and end of fuzzy region in the AOF ((region that contains mix of v and v+1 log entries).
• Fix AOF replay to correctly process single-key operations and transactions in fuzzy region. Specifically, we process all (v) ops and txns in the fuzzy region, then take a checkpoint, then process all the stored (v+1) ops and txns in the fuzzy region.

Notes:

• Recovery and cluster logic is left unmodified, they assume checkpoints are separate across main and object stores. This allows the PR to remain backward compatible.

[TedHartMS]

approved with one comment